(a) Importance of PPS--Achievement of sharp, clean images in inkjet printing requires that the distance between each inkjet printhead or "pen" 30, 30' (FIG. 1) and the paper or other printing medium 2 be controlled very stringently. As is well known, in an inkjet printer a central processor 80 selectively fires the pen nozzles during scanning 32, 32', to form the desired images on the print medium. (In the drawing the printheads are represented conceptually as pens 30 ejecting ink 31 while traveling leftward 32, and also as pens 30' ejecting ink 31' while traveling rightward 32'. These separate representations in the drawing represent the same, identical pens but merely scanning in opposite directions.)
The spacing between the pen and the print medium is called pen-to-paper spacing (or printhead-to-printing-medium spacing) and abbreviated "PPS". It is a critical parameter because the quality of a printed image is greatly affected by relatively small changes in PPS. One reason is that the character of inkdrops 31, 31' in flight--and the resulting ink-spot size--change dramatically with distance of flight.
Another reason is that the rapid scanning motion 32, 32' of inkjet pens, during inkdrop ejection, interacts with the PPS to modify the accuracy of ink-spot placement. The combined effect is a great variation in the size and registration of ink spots formed on the printing medium by pens ejecting ink of different colors--and even by the same pen when traveling in opposite directions 30, 30'.
In modern inkjet systems the pens 30, 30' are coupled 1, 3 to an optoelectronic sensor 37 that monitors fiduciary markings along a scale 38, sending electrical signals 39 to the central processor 80 for development of position and speed information. Some but not all inkjet systems servocontrol the scanning speed to make it constant at all times when the pens are ejecting ink to form an image. To the extent that speed variation is permitted, yet another variable function of the PPS is introduced--i.e., as between pens traveling in the same direction but at different speeds.
Because of these several sensitivities to small PPS changes, in systems with which we are most familiar the overall permissible variation of PPS for optimum print quality is less than .+-.0.4 mm (about .+-.0.015 inch).
(b) Economics of PPS control--Special devices and techniques are commonly used, and heretofore have been considered necessary, to control this parameter in a high-volume manufacturing environment. Two common requirements, in particular, are for special adjusting tools--used in a painstaking, time-consuming procedure at an initial measurement/adjustment station on the assembly line--and also a later PPS verification station further along the line. Some printers require a special fixture to measure and adjust PPS. The fixture is complex and must be closely monitored and maintained.
All such provisions are costly in terms of initial hardware capital equipment to establish each new production line in different parts of the world, and also in terms of ongoing labor to staff and supervise these production stations. As will now be clear to a person skilled in this field, PPS control heretofore has been expensive. It has furthermore been less than completely successful.
(c) Mechanics of PPS control heretofore--During printing, inkjet pens are held and transported across a printing-medium page by a scanning carriage 20 that slides on bushings along a support-and-guide rod 6, usually called the "slide-rod". (In FIG. 1 the carriage and rod are represented in common simply by a dashed line 20, 6.)
The rod is supported by a chassis element 10. The central processing unit 80 provides position and speed signals 34 to a motor 35, which operates an endless belt 36 to drive the pen-holding carriage 20 along the rod 6.
The printing medium 2, meanwhile, typically is held and located to a chassis element 10' by a platen 7. In the drawing, the platen is represented for conceptual purposes as a classical typewriter-style rotary platen--with a shaft 51 that is rotatably mounted to the chassis element 10' (as symbolized at right)--and the processor 80 provides electrical signals 53 to a motor 52 that drives the shaft 51. While our invention encompasses such a system, we prefer a different kind of platen and printing-medium drive as will be seen.
In some devices the chassis elements 10, 10' locating the pen carriage and platen respectively have been separate elements fastened together. In other devices they have been neighboring portions of a common chassis.
PPS naturally is controlled by the distance between the pens 30, 30' and the platen 7, and is subject to variation on account of accumulated tolerances between the pens and platen. Key to this accumulation of errors is the relative positioning of the slide-rod and platen to their respective supporting chassis elements, as well as the relative positioning of those chassis elements to each other.
Most inkjet systems heretofore have been designed with an incorporated adjustment mechanism to enable all the accumulated errors to be, in effect, removed by an assembly worker on a production line. In purest principle such adjustment may be taken in the relative positioning of either the platen to the chassis, or the pen (particularly the slide-rod) to the chassis.
The platen, however, is subject to other relatively rigorous constraints by virtue of the interaction of the printing medium with other components in the print-medium advance path. Therefore in many systems adjustment at the platen is disfavored.
Typically therefore it is the slide-rod that has been secured to the chassis through the intermediary of an adjustment system--which heretofore has provided either multistep or continuous (sometimes called "infinite") adjustment. When such an adjustment system has not yet been adjusted or secured at any particular adjustment position, the PPS is typically free to vary a great deal. In many systems it can vary by several times the acceptable variation of PPS.
Consequently satisfactory operation relies totally upon correct adjustment, stabilization and performance of the adjusting system. Furthermore, tolerances contributed in the adjustment devices themselves can consume the entire acceptable PPS variation.
Stabilization of PPS adjustments in general has been accomplished using fasteners that directly lock an adjustable element in place. Torque-type fasteners are especially difficult to control in a PPS system, because every time a fastener is driven, torque transmitted throughout the printing-machine structure inevitably affects PPS. This is particularly important in view of the small (.+-.0.37 mm) window within which inkjet print quality is optimized.
Prior systems are also characterized, in general, by relatively high parts count--a relatively large number of standard fasteners as well as special fittings. It is well known that each incremental fastener or other part to be interconnected correctly in an assembly-line environment tends to add significantly and undesirably to production cost.
(d) Examples of prior systems--The following printers all employ a slide-rod and a carriage assembly, in addition to the parts mentioned below.
A certain portable Canon printer has a sheetmetal chassis, three screws, two springs, and an adjustable bar. A screw is used to provide axial support of the slide-rod. Driving this screw inevitably moves the rod and affects PPS. The Canon PPS adjustment also uses two screws to secure an adjusting rod in place: torquing down these screws shifts the PPS adjustment from its intended position.
As another example, a certain Epson printer has six sheetmetal chassis, more than ten screws, and two adjustable caps. There are so many chassis parts (six) and associated screws to interconnect them, as well as screws in each of two adjustable cap parts, that substantial distortion appears unavoidable. This would suggest a high rate of intervention to adjust PPS. That adjustment is performed by rotation of plastic caps that fit on the ends of the slide-rod and connect to the chassis via a hub and the two screws mentioned above.
Still another example is a printer from the Hewlett Packard Vancouver Division, which has three sheetmetal chassis, four screws and two adjustable caps. The four screws are used to secure the three chassis members together. The associated deformation would affect PPS.
Critical components of that system are the printer chassis 110 (FIG. 20), left endcap 140, endcap pivot point 142, and two-sided cam 118 for shifting the slide-rod location 116--and of course corresponding parts (not shown) at the right end of the chassis 110. This device offers continuous adjustment, so that PPS can be set to exactly the optimal desired value.
Torquing a lock-screw through the locking hole 146 provided in the cam plate 140 to secure the adjustment, however, is likely to disturb the slide-rod position 116 as well as introducing stress and offset into the mechanism generally. As will be understood, it is not our purpose to unduly derogate the illustrated system--as that system is itself not only useful but also a substantial improvement and advance relative to the general state of the prior art--but rather only to point up areas where room for improvement is present in theory. This illustrated HP system is discussed further in section (f) below.
The overall parts count is nine for the Canon, twenty for the Epson, and eleven for the illustrated Hewlett Packard printer.
Yet another Hewlett Packard product, the DeskJet Portable, has taken an opposite approach, namely provision of no adjustment capability at all--thereby wholly avoiding the considerable cost of parts and labor for adjustment. A drawback of this approach is that some small number of production machines must be scrapped or reworked, at very disproportionately high cost.
As shown by the foregoing discussion, heretofore some relatively advanced features have been found only in portable units--of both the Canon and the Deskjet product lines. True desktop machines, by contrast, have been denied the benefits of both a common chassis support for the platen and slide-rod, and positive (though unadjustable) mating of the slide-rod to the chassis. These differences may arise mainly from the lower cost and greater ruggedness required of a portable printer, rather than greater sophistication in design.
(e) Production tooling--In the above-mentioned Canon product, two screws secure an adjusting bar in position. Proper placement of that bar must be accomplished through production-line tooling. Essentially another part, the PPS tool, is introduced that requires careful control and calibration--and which in turn add more variation to the adjustment.
We do not know what tooling may be needed for PPS adjustment in assembly of the above-discussed Epson printer. In the illustrated prior Hewlett Packard printer two rotating plastic caps are positioned through use of assembly tooling, primarily a measurement nest that requires a tool to comfortably rotate the caps.
(f) Chassis design--In this regard the Canon product represents a relatively advanced design. It has a single sheetmetal member to hold all contributors to PPS variation and adjustment, and its chassis supports both the slide-rod and the platen.
The Epson unit, in contrast, employs so many chassis parts (six) that the worst-casing loop for PPS tolerance becomes unnecessarily cluttered. Contributors to PPS variation are not held in reference to each other by a single chassis part. In addition, the slide-rod is located by the caps that rotate to adjust PPS, rather than by a chassis part; this needlessly introduces the component variations of the caps themselves into the tolerance loop for both default and adjusted PPS. It also leaves the rod supported by the cap, risking failure in abusive situations such as mechanical shock and vibration.
The prior HP printer, though not to the same extent as the Epson product, uses multiple chassis parts (three). Contributors to PPS variation are not held in reference to each other in a single chassis part. It too includes the adjustable cap parts in the tolerance loops. As can be seen from the operating relationships of this mechanism (FIG. 20), accuracy of the resulting slide-rod positioning is affected by dimensional instabilities in the cam 118, 119 radii. (If the scale fiducial markings are treated as absolute values, rather than by use of an independent standard measuring device, then the slide-rod position is affected by tolerances in the cam and scale 119, too.) In addition, as mentioned earlier, torquing of a fastener in the securing hole 146 is likely to displace the setting from the chosen value.
(g) Carriage assembly and orientation--Bushings used to enhance sliding motion of the carriage along the slide-rod are a source of PPS error. This error stems in part from tolerances in bushing dimensions, but more importantly from misalignment, other mispositioning, and deformation that all arise as bushings are pressed into the carriage body.
The Canon configuration perhaps represents an effort to avoid imprecision contributions from these sources by using no bushings, although naturally the carriage-molding process is itself subject to imprecision. It also has, near the top of the carriage, a more complex secondary support--that could be subject to greater variation and thus affect PPS.
Also the PPS adjustment in the Canon configuration rotates the carriage--and therefore the printhead nozzle plate. Print-quality errors could result without the addition of some sort of calibration describing the rotation of the nozzle plate, since one end of the plate is further from the paper than the other. No such calibration is apparent in the product, and would be difficult on an assembly line.
The illustrated prior Hewlett Packard printer is manually assembled. It is therefore subject to additional tolerances which also are typically difficult to characterize and counteract.
(h) Conclusion--In offering the foregoing comparative discussion of existing PPS-control configurations it is our intention only to highlight some important considerations, and not to criticize earlier efforts as these have created worthwhile and eminently usable consumer products. Nevertheless some of the limitations discussed have continued to impede achievement of uniformly excellent inkjet printing at an optimal cost. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.